Several different types of light sources or lamps have been incorporated into optical scanners in order to illuminate the item being scanned. Among other criteria, the choice of lamp type and style is often based upon cost, size, lamp life, light intensity, output spectrum, power requirements, and turn-on time. Such lamps include cold cathode fluorescent, hot cathode fluorescent, xenon, and light emitting diode (LED) light sources.
In modem scanner systems, especially those using the more popular cold cathode fluorescent lamps, the warm-up time for the scanner's light source is relatively long. For greater productivity, the scanner should be immediately available for use at any time. In a practical sense, the only technique by which this objective can be met is to leave the light source turned on all the time. However, if the light-source is on continuously, excessive power consumption and a large reduction in lamp operational life will result. As an example, cold cathode fluorescent lamps have an average lamp life of between 10,000 and 15,000 hours prior to burn-out. If a cold cathode lamp is on for only 1,000 hours per year, the operational life of the lamp is approximately 10 to 15 years, and even at 2,000 hours per year, it is 5 to 7.5 years. However, if on full time, the operational life of the lamp will be reduced to only 14 to 20 months.
The intensity and spectral balance in the light output from a lamp typically depends on its temperature. Therefore, light output is constantly varying while the lamp is warming up. Without elaborate compensation schemes, the user's first scan can be either of poor quality or can require a noticeable delay before it starts.
Some techniques exist for reducing operator perceived warm-up time for the lightsource, while minimizing degradation in lamp operational life. However, all previous techniques have some disadvantages associated with them. For example, Mustek 1200 IIIEP scanners use a mechanical switch to sense the position of the document lid. Warm-up of the light-source is initiated when the mechanical switch senses that the document lid is no longer closed. The time which the user consumes in document placement and other activities prior to pushing the start button reduces the perceived wait time. Disadvantages of this system are the requirement for a cable connecting the mechanical switch to the electronics controlling the light-source and the reliability of mechanical switches.
Essentially immediate scanning can be accomplished with some combination of the following techniques: (1) use of a stand-by current to keep the lamp near its long term operating temperature when the machine is not in use and (2) use of an instant-on lamp. A disadvantage of the stand-by current system is that significant power is still consumed during long periods of idle time. Also, if the idle current is turned off for some reason, such as after a long period of non-use, the next user will have to wait for the full warm-up to take place. Disadvantages of the instant-on lamp are its relatively high cost and the fact that the spectral output may not match that of the sensitivity of the light-sensor.
Other possible solutions include (1) sensing the light-source output while the page is being scanned and adjusting the power supplied to the lamp in order to hold the lamp output constant and (2) sensing the lamp output while the page is being scanned, and adjusting the sensitivity of the light sensor. A disadvantage of sensing the lamp output while the page is being scanned and adjusting power supplied to the lamp is that the lamp output is limited by initial lamp temperature, so the entire scan must be run at a slower speed, corresponding to the low lamp output at the beginning. This method also requires stability of the spectral output of the lamp, which takes a significant amount of time, as well as a light monitor window with associated additional detection pixels. Sensing the lamp output while the page is being scanned and adjusting the sensitivity of the light sensor has the disadvantage, once again, that the entire scan is run at slow speed, corresponding to the low lamp output at the beginning of the scan. Profile stability and the requirement for a light monitor window with associated additional pixels are issues here also.
Thus, there is a significant need to improve the scanner system so as to reduce, at least, the operator perceived warm-up time for the light-source, while minimizing degradation in lamp operational life. In particular, there is a need for a system to provide power to a scanner lamp that takes only approximately a minute or so to approach its long-term temperature after the operator has completed other activities prior to scanning.